The story takes place forty years after a mysterious occurrence causes the residents of Paradigm City to lose their memories.

The series follows Roger Smith, Paradigm City's top Negotiator. He provides this much needed service with the help of an gynoid named R. Dorothy Wayneright and his butler Norman Burg. When the need arises, Roger calls upon Big O, a giant relic from the city's history that may hold the key to its future.